The invention relates to a multimirror device for rotating the polarization of an electromagnetic wave, especially a light wave. It also relates to the application of such a device to an image-projection apparatus of the liquid-crystal type.
In many technologies it is necessary to rotate the polarization of an electromagnetic wave. The following description will be limited to the example of a fixed-image or moving-image projector in which the images are generated in a component comprising liquid crystals of the nematic type. It is known that such liquid crystals must be illuminated by light linearly polarized in a defined direction with respect to the axes of the liquid crystal.
In order to produce this linearly polarized light, an unpolarized light source is used together with a polarizing beam splitter which delivers, in a first direction, polarized light suitable for illuminating the liquid-crystal component and, in a perpendicular direction, light having the crossed polarization, i.e. light whose polarization vector is rotated through 90xc2x0 with respect to the light in the first direction. In order for all, or most, of the energy delivered by the light source to be used to illuminate the liquid-crystal component, use is often made of a polarization-rotating device which rotates the polarization of the light received through 90xc2x0 into the crossed polarization.
There are two known ways of rotating the polarization through 90xc2x0.
The first way consists in providing a plate called a quarter-wave plate, or xcex/4, plate which converts the crossed linear polarization into a circular polarization in one direction and a mirror which reflects the circularly-polarized light. The reflected signal has a circular polarization in the other direction and, after it has passed through the quarter-wave plate, the linear polarization of this signal is perpendicular to the direction that it had on entering this quarter-wave plate. Thus, on leaving the quarter-wave plate, after reflection off the mirror, the light has the linear polarization of suitable direction. Such a device, in which the incoming and outgoing beams have the same direction, is relatively compact; this is why it is widely used. However, it has the drawback of operating correctly only for a single wavelength and for a defined direction.
The second known way consists in providing an arrangement of two mirrors forming the faces of a total-reflection prism, the light striking these faces at an angle of incidence of 45xc2x0. The orientation and the arrangement of these mirrors are such that the polarization is in the same direction after reflection off one of the mirrors and is in the perpendicular direction after reflection off the other mirror. Such a polarization-rotating device is almost wavelength-insensitive. However, it is annoyingly bulky. In addition, given that the outgoing beam of this device is perpendicular to the incoming beam, it does not fit well into the usual applications which require the incoming beam and the outgoing beam to be in the same direction.
The invention provides a polarization-rotating device which has the advantages of the two approaches known hitherto, but without their drawbacks. The device according to the invention is therefore wavelength-insensitive, is not very bulky, and the output beam is in the same direction as the input beam.
This device, in which the output beam is in the same direction as the input beam, is characterized in that it comprises at least one combination of three mirrors, the light being reflected off them each time at an angle of 45xc2x0, and in that the incident beam and the mirrors are arranged in such a way that two mirrors preserve the direction of polarization and the third rotates this polarization through 90xc2x0.
In the preferred embodiment of the invention, a plurality of three-mirror combinations is provided, each combination forming a regularly repeated pattern and the patterns all having identical shapes and sizes. It is particularly advantageous for these patterns to form reliefs on one face of a sheet of transparent material. In this case, the polarization-rotating device may be produced by moulding, for example by moulding a transparent plastic such as an acrylic material.
It will be noted that since the transparent materials normally used have a refractive index generally between 1.4 and 1.7, the total reflection occurs at and beyond an angle of incidence of approximately 40xc2x0. Under these conditions, incidence at 45xc2x0 corresponds to total reflection, i.e. without loss.
Preferably, the patterns are contiguous and, in projection on a plane perpendicular to the incident rays, these patterns entirely fill a surface without any discontinuity (interruption).